The invention relates to an aqueously dispersible polyurethane. It further relates to a process of preparation thereof, and a method of use of the dispersion thus prepared.
Aqueous dispersions of polyurethanes have been known, i. a., from U.S. Pat. No. 5,334,651. These polyurethanes are based on reaction products of polyfunctional isocyanates, polyols having a number average molar mass of at least 400 g/mol, and compounds having at least two groups which are reactive towards isocyanate groups, and at least one group capable of anion formation, which are reacted to form an isocyanate-functional prepolymer, and which prepolymer is reacted in a second step with at least one of low molar mass polyols, and compounds which have one active hydrogen atom, or more than one active hydrogen atom wherein these said hydrogen atom differ in reactivity towards isocyanate groups. These polyurethanes are hydroxy-functional at least due to the reaction of the second step, and aqueous dispersions thereof can be cured with isocyanate crosslinking agents to give tough and resilient films.
Coating films prepared from these polyurethanes can still be improved particularly with respect to their hardness and solvent resistance.